1. Field of the Invention
The present invention relates to a connection structure between a single-mode optical fiber and a dispersion compensating optical fiber, and more specifically, relates to a connection structure between a single-mode optical fiber and a dispersion compensating optical fiber having a negative dispersion slope.
2. Description of Related Art
A transmission line which combines a single mode optical fiber such as one for use at 1.3 μm and a dispersion compensating optical fiber for compensating the chromatic dispersion thereof, has heretofore been put to practical use.
The single mode optical fiber has a relatively large effective core area (Aeff), and hence nonlinear effects are suppressed. However, if a 1.55 μm band or the like is designated as the wavelength region for use, chromatic dispersion increases. Therefore, a low loss transmission line can be built up by compensating the chromatic dispersion with the dispersion compensating optical fiber.
Japanese Patent No. 2,951,562 discloses a structure for connecting a normal single mode optical fiber and a dispersion compensating optical fiber having a so-called W-type refractive index distribution shape for compensating the chromatic dispersion of the single mode optical fiber, via an intermediate optical fiber interposed therebetween.
In this structure, the mode field diameter of the intermediate optical fiber is made to have substantially the same value as the mode field diameter of the dispersion compensating optical fiber. Also, the mode field diameter of this intermediate optical fiber on the single mode optical fiber side is expanded so as to match the mode field diameter of the single mode optical fiber.
As a result, the joint loss between the intermediate optical fiber, the dispersion compensating optical fiber and the single mode optical fiber is reduced. Expansion of the mode field diameter is performed by heating the end of the intermediate optical fiber to diffuse the dopant such as germanium added to the core thereof.
Recently, with the development of wavelength multiplex transmission, a dispersion compensating optical fiber which can compensate not only the chromatic dispersion of the single mode optical fiber but also the dispersion slope has been developed.
The dispersion slope is an inclination in the graph when the wavelength is plotted on the X-axis and the chromatic dispersion is plotted on the Y-axis, and the normal single mode optical fiber has a positive dispersion slope. Therefore, if the dispersion slope of the single mode optical fiber is compensated by the dispersion compensating optical fiber having a negative dispersion slope, a flat chromatic dispersion characteristic can be obtained in a relatively wide wavelength region.
The normal single mode optical fiber referred to herein is assumed to be one normally used for propagating an optical signal, such as one for use at 1.3 μm and a dispersion shifted optical fiber.
Moreover, the dispersion compensating optical fiber is preferably one capable of single mode propagation.
However, the dispersion compensating optical fiber having such a negative slope has a different refractive index distribution shape from that of the dispersion compensating optical fiber of a type for compensating only the chromatic dispersion which has been heretofore proposed. Hence even if the connection structure is formed based on the mode field diameter as disclosed in Japanese Patent No. 2,951,562, the joint loss cannot be reduced. Particularly, when fusion splicing is performed, there is a tendency for the increase in joint loss to become conspicuous.
As the dispersion compensating optical fiber, there has been provided one having an expanded effective core area, in order to prevent deterioration in the transmission quality due to the nonlinear effect which occurs therein. In this dispersion compensating optical fiber having an expanded effective core area, there is a tendency for the increase in joint loss due to fusion splicing to further increase.